A pressure mediator includes a base body, or platform, and a separating membrane, or diaphragm, which is secured with its edge to the base body, such that the separating membrane can be placed with its first, platform-far face in contact with the first medium. With its second, platform-near face, the membrane forms with the platform a pressure chamber, which can be filled with the second medium. The pressure chamber has a pressure chamber opening, through which the pressure can be transferred by means of the second medium. The second medium is usually an incompressible liquid, especially a hydraulic oil.
Separating membranes usually are provided with a relief of concentric waves, which enable a sufficiently large membrane stroke for accommodating a variable volume of the transfer liquid in the pressure chamber. The surface of the base body in the pressure chamber is usually embodied as a membrane bed, whose relief largely matches that of the separating membrane, so that the separating membrane can lie flush against the membrane bed in the case of overload. The separating membranes are usually pressed against the membrane bed, in order to emboss them with the desired relief.
In the case of pressure mediators for use with corrosive media, the separating membranes are preferably made of a corrosion-resistant material, especially a corrosion-resistant metal, or a corrosion-resistant alloy. Separating membranes of tantalum are preferably used for this purpose. Since the base bodies are usually made of VA-steel, problems arise in practice due to the different coefficients of thermal expansion of tantalum and steel, that for tantalum being 6.5 10−6/K for tantalum and that for V2A-steel being about 16 10−6/K.
If one assumes, for example, that the pressure mediator will be used over a temperature range of about 300° K, for example between 230° K and 530° K, then a separating membrane of tantalum will expand over this temperature range by only about 0.21%, while the expansion of the V2A-platform will amount to 0.48%. The difference in the thermal expansion over this temperature range thus amounts to 0.27%.
For a pointed illustration of the problem resulting therefrom, first assume that the membrane is planar. A planar, circular, separating membrane, welded stress-free onto the base body at the upper temperature limit, would exhibit an equilibrium position having a deflection of about 6.3% of the radius at the lower temperature limit. To be more precise, there would be two equilibrium positions, lying+/−6.3% outside of the plane of the membrane edge. Such a bistable membrane would clearly be unusable for a sensor. The stress-free welding of a planar separating membrane at the lower temperature limit would, it is true, prevent the problem of the deflection due to different coefficients of thermal expansion, but, upon warming, large radial tensile stresses would arise due to the different coefficients of thermal expansion. This would compromise the calibration of the membrane.
The mentioned separating membranes with concentric waves ameliorate the described problem, because, on the one hand, the waves contain sufficient radial length reserves to accommodate the differences in coefficient of thermal expansion, and, on the other hand, the forming of the waves leads to a moderate residual radial tensile stress, which at least lessens the deflection of the membrane in the case of a contraction of the base body. However, at low temperatures, deflections still occur.